1. Field of the Invention
This invention relates generally to reducing interference in a satellite receiving system that is caused by other satellite signals being received and processed by the system.
2. Prior Art
Satellite broadcast signals are made spatially orthogonal by a combination of antenna design and signal timing in order to reuse the frequency spectrum of the signals. Two signals that share an overlapping frequency are polarized when transmitted to allow separation at the receiver. Due to atmospheric conditions, antenna design limitations, signal dish and antenna alignment, as well as signal isolation (internal to the low-noise block converter (LNB) or switch) it is inevitable that the two polarizing mechanisms of the signals occupying the same frequency will interfere with each other. A common term for this is cross polarization interference.
U.S. Pat. No. 5,760,740 to Blodgett, issued Jun. 2, 1998, entitled “Apparatus and method for electronic polarization correction,” incorporated herein by reference, describes a circuit arrangement that applies the antenna output signals to a combiner at an optimized relative amplitude and phase to substantially cancel the received cross polarized energy of a second electromagnetic signal.
U.S. Pat. No. 4,106,015 to Beguin et al., issued Aug. 8, 1978, entitled “Radar system with circular polarized transmission and adaptive rain depolarization compensation,” incorporated herein by reference, describes an apparatus for detecting variations of phase between two received channels and automatically adjusting amplitude and phase of at least one of the two channels in order to compensate for the depolarization effects of rain.
From the perspective of the receiving and transmitting antenna, the signals are polarized with a vertical only or a horizontal only component. The wave of each signal travels from transmitter to receiver in the exact same physical orientation of which it left the transmitter. An analogy is in a simple sunlight filtering using polarized lenses where the horizontal light is seen by orienting a polarized lens so the horizontal component of light can be seen and the vertical component is removed. Separately, when another polarized lens is rotated 90 degrees relative to the previous lens, only the vertical light transmitted. A second receiving lens that is oriented the same as the filtering lens will reject the opposite polarization and pass only the desired polarization. The selectivity of the antenna in a satellite system using horizontal/vertical polarizing is similar to the light corollary.
Antenna pointing error, or in other words, the receiving antenna angle misalignment, degrades isolation by the factor 20*LOG (angle of misalignment).
In addition to horizontal and vertical types of polarization, there are right-hand and left-hand circular polarizations where a given signal occupies both vertical and horizontal polarization simultaneously, the difference being the sequencing between the vertical and horizontal polarizations. The vertical signal lags or leads the horizontal in both time and physically in space to determine the direction of the polarization.
After being depolarized at the antenna, the signals are converted to electrical representations denoted herein as signal A and B, where A is the electrical signal with the majority of the signal being originally right-hand polarized and B is from the left-hand polarized signal. Or equivalently, in a separate system, signal A is the electrical signal with the majority from a horizontal polarization and B is from a vertical polarization.
During times of intense raining (rain fade), attenuation increases and isolation between right and left-hand polarized signals degrades further.
Removal of the cross contamination between the two signal polarizations is needed to correct for the degradation of isolation occurring during transmission and reception.
In addition to cross contamination between signal polarizations, signal channels can experience interference from other sources. An outdoor unit may incorporate two or more LNBs to receive signals from multiple satellites positioned in different orbital slots. The directivity of the receiving antenna focuses the signal primarily from one satellite to one LNB. The radio frequency (RF) or intermediate frequency (IF) signals from channels that occupy overlapping frequencies from other satellites can cause interference. This can be called cross-satellite interference or leakage. Interference can come from other sources in the receiving unit. The interfering signal may be from transponder channels that are aligned with the wanted signal or that have a frequency offset.
The LNB output drives an indoor unit, which can be a set-top, media center, or other device that tunes, demodulates, and decodes the video programs.
Further processing of the desired signal can be adversely affected by unwanted interference from same satellite cross polarization interference or interference by a signal from another satellite. There exists a need for a cost effective means of canceling the effects of interfering RF or IF signals present in a received satellite signal.